Cellulosic ethanol is a type of biofuel produced from lignocellulose, a structural material that comprises much of the mass of plants. It is composed mainly of cellulose, hemicellulose and lignin. Corn stover, switchgrass, miscanthus and woodchip are some of the more popular cellulosic materials for ethanol production. Cellulosic ethanol is chemically identical to ethanol from other sources, such as corn starch or sugar, but has the advantage that the lignocellulose raw material is highly abundant and diverse. However, it differs in that it requires a greater amount of processing to make the sugar monomers available to the microorganisms that are typically used to produce ethanol by fermentation.
There are at least two methods of production of cellulosic ethanol (see "Production methods", below):
- Cellulolytic method: hydrolysis followed by fermentation of the generated free sugars.
- Gasification, which produces synthesis gas that can be converted to ethanol by fermentation or thermochemical catalysis (e.g., the Fischer-Tropsch process).
Neither process generates toxic emissions when it produces ethanol.
Cellulosic ethanol production currently exists at "pilot" and "commercial demonstration" scale, including a plant in China engineered by SunOpta Inc. and owned and operated by China Resources Alcohol Corporation that is currently producing cellulosic ethanol from corn stover (stalks and leaves) on a continuous, 24-hour per day rate.
According to US Department of Energy studies conducted by the Argonne Laboratories of the University of Chicago, one of the benefits of cellulosic ethanol is that it reduces greenhouse gas emissions (GHG) by 85% over reformulated gasoline. By contrast, starch ethanol (e.g., from corn), which most frequently uses natural gas to provide energy for the process, reduces GHG emissions by 18% to 29% over gasoline. Sugar ethanol is cheaper than corn ethanol. Cellulosic ethanol from sugarcane bagasse, reduces greenhouse gas emissions by as much as cellulosic ethanol. In both cases the waste lignin becomes fuel to provide the energy for the process with some excess to provide electricity for the grid.
- 1 History
- 2 Production methods
- 3 Economic importance and viability
- 4 Prominent Cellulosic Ethanol Researchers
- 5 See also
- 6 References
- 7 External links
In April 2004, Iogen Corporation, a Canadian biotechnology firm, became the first business to commercially sell cellulosic ethanol, though in very small quantities. The primary consumer thus far has been the Canadian government, which, along with the United States government (particularly the Department of Energy's National Renewable Energy Laboratory), has invested millions of dollars into assisting the commercialization of cellulosic ethanol.
Another company which appears to be nearing commercialization of cellulosic ethanol is Spain's Abengoa Bioenergy . Abengoa has and continues to invest heavily in the necessary technology for bringing cellulosic ethanol to market. Using process and pre-treatment technology from SunOpta Inc.(NASDAQ: STKL), Abengoa is building a 5 million gallon cellulosic ethanol facility in Spain and has recently entered into a strategic research and development agreement with Dyadic International, Inc. (AMEX: DIL), to create a new and better enzyme mixtures which may be used to improve both the efficiencies and cost structure of producing cellulosic ethanol.
On December 21, 2006, SunOpta Inc. announced a Joint Venture with GreenField Ethanol, Canada's largest ethanol producer. The joint venture will build a series of large-scale plants that will make ethanol from wood chips, with SunOpta Inc. and GreenField each taking 50% ownership. The first of these plants will be 10 million gallons per year, which appears to be the first true "commercial scale" cellulosic ethanol plant in the world. Under 1 million gallons per year (MMgy) is considered "Pilot Scale", greater than 1 MMgy but less than 10 MMgy is defined as "commercial demonstration", while a plant that produces 10 MMgy per year or greater is true "commercial scale". Despite the multiple commercial demonstration cellulosic ethanol plants SunOpta has been involved with, media reports continue to state that cellulosic ethanol is an unproven, "experimental" technology. The 10 MMgy SunOpta/GreenField cellulosic ethanol plant is intended to demonstrate that large-scale cellulosic ethanol is commercially viable immediately.
President Bush, in his State of the Union address delivered January 31, 2006, proposed to expand the use of cellulosic ethanol. In his State of the Union Address on January 23, 2007, President Bush announced a proposed mandate for 35 billion gallons of ethanol by 2017. It is widely recognized that the maximum production of ethanol from corn starch is 15 billion gallons per year, implying a mandated production of some 20 billion gallons per year of cellulosic ethanol by 2017. Bush's plan includes $2 billion funding for cellulosic ethanol plants, with an additional $1.6 billion announced by the USDA on January 27, 2007.
In March 2007, the US government awarded $385 million in grants aimed at jumpstarting ethanol production from nontraditional sources like wood chips, switchgrass and citrus peels. Half of the six projects chosen will use thermochemical methods and half will use cellulosic ethanol methods .
The American company Range Fuels announced in July 2007 that it was awarded a construction permit from the state of Georgia to build the first commercial-scale 100-million-gallon-per-year cellulosic ethanol plant in the United States .
There are two ways of producing alcohol from cellulose:
- Cellulolysis processes which consist of hydrolysis on pretreated lignocellulosic materials followed by fermentation and distillation.
- Gasification that transforms the lignocellulosic raw material into gaseous carbon monoxide and hydrogen. These gases can be converted to ethanol by fermentation or chemical catalysis.
They both include distillation as the final step to isolate the pure ethanol.
Cellulolysis (Biological approach)
There are four or five stages to produce ethanol using a biological approach:
- A "pretreatment" phase, to make the lignocellulosic material such as wood or straw amenable to hydrolysis,
- Cellulose hydrolysis (cellulolysis), to break down the molecules into sugars;
- Separation of the sugar solution from the residual materials, notably lignin;
- Microbial fermentation of the sugar solution;
- Distillation to produce 99.5% pure alcohol.
Although cellulose is the abundant resource in plant materials, its susceptibility has been curtailed by its rigid structure. As the result, an effective pretreatment is needed to liberate the cellulose from the lignin seal and its crystalline structure so as to render it accessible for a subsequent hydrolysis step . By far, most pretreatments are done through physical or chemical means. In order to achieve higher efficiency, some researchers seek to incorporate both effects .
To date, the available pretreatment techniques include acid hydrolysis, steam explosion, ammonia fiber expansion, alkaline wet oxidation and ozone pretreatment . Besides effective cellulose liberation, an ideal pretreatment has to minimize the formation of degradation products because of their inhibitory effects on subsequent hydrolysis and fermentation processes . The presence of inhibitors will not only further complicate the ethanol production but also increase the cost of production due to entailed detoxification steps. Even though pretreatment by acid hydrolysis is probably the oldest and most studied pretreatment technique, it produces several potent inhibitors including furfural and hydroxymethyl furfural (HMF) which are by far regarded as the most toxic inhibitors present in lignocellulosic hydrolysate . In fact, Ammonia Fiber Expansion (AFEX) is the sole pretreatment which features promising pretreatment efficiency with no inhibitory effect in resulting hydrolysate 
The cellulose molecules are composed of long chains of sugar molecules of various kinds. In the hydrolysis process, these chains are broken down to free the sugar, before it is fermented for alcohol production.
There are two major cellulose hydrolysis (cellulolysis) processes: a chemical reaction using acids, or an enzymatic reaction.
In the traditional methods developed in the 19th century and at the beginning of the 20th century, hydrolysis is performed by attacking the cellulose with an acid. Dilute acid may be used under high heat and high pressure, or more concentrated acid can be used at lower temperatures and atmospheric pressure. A decrystalized cellulosic mixture of acid and sugars reacts in the presence of water to complete individual sugar molecules (hydrolysis). The product from this hydrolysis is then neutralized and yeast fermentation is used to produce ethanol. As mentioned, a significant obstacle to the dilute acid process is that the hydrolysis is so harsh that toxic degradation products are produced that can interfere with fermentation. Concentrated acid must be separated from the sugar stream for recycle (simulated moving bed (SMB) chromatographic separation for example) to be commercially attractive.
BlueFire Ethanol Fuels utilizes post-sorted MSW, rice and wheat straws, wood waste and other agricultural residues and implements significant proprietary improvements to concentrated acid hydrolysis. The Technology is unique in that, for the first time, it enables widely available cellulosic materials, or more commonly, biomass, to be converted into sugar in an economically viable manner, thereby providing an inexpensive raw material for fermentation or chemical conversion into any of a hundred different specialty and/or commodity chemicals. In February of 2007, BlueFire Ethanol was among 6 companies that received a grant from the US Department of Energy for $40M to promote development of cellulosic ethanol refineries.
Cellulose chains can be broken into glucose molecules by cellulase enzymes.
This reaction occurs at body temperature in the stomach of ruminants such as cows and sheep, where the enzymes are produced by bacteria. This process uses several enzymes at various stages of this conversion. Using a similar enzymatic system, lignocellulosic materials can be enzymatically hydrolyzed at a relatively mild condition (50oC and pH5), thus enabling effective cellulose breakdown without the formation of byproducts that would otherwise inhibit enzyme activity. By far, all major pretreatment methods, including dilute acid pretreatment, require enzymatic hydrolysis step to achieve high sugar yield for ethanol fermentation .
Various enzyme companies have contributed significant technological breakthroughs in cellulosic ethanol through the mass production of enzymes for hydrolysis at competitive prices.
Iogen Corporation is a Canadian producer of enzymes for an enzymatic hydrolysis process that uses "specially engineered enzymes". The raw material (wood or straw) has to be pre-treated to make it amenable to hydrolysis.
Another Canadian company, SunOpta Inc. markets a patented technology known as "Steam Explosion" to pre-treat cellulosic biomass, overcoming its "recalcitance" to make cellulose and hemicellulose accessible to enzymes for conversion into fermenatable sugars. SunOpta designs and engineers cellulosic ethanol biorefineries and its process technologies and equipment are in use in the first 3 commercial demonstration scale plants in the world : Verenium (formerly Celunol Corporation)'s facility in Jennings, Louisiana, Abengoa's facility in Salamanca, Spain, and a facility in China owned by China Resources Alcohol Corporation (CRAC). The CRAC facility is currently producing cellulosic ethanol from local corn stover on a 24-hour a day basis utilizing SunOpta's process and technology.
Genencor and Novozymes are two other companies that have received United States government Department of Energy funding for research into reducing the cost of cellulase, a key enzyme in the production of cellulosic ethanol by enzymatic hydrolysis.
Other enzyme companies, such as Dyadic International, Inc. (AMEX: DIL), are developing genetically engineered fungi which would produce large volumes of cellulase, xylanase and hemicellulase enzymes which can be utilized to convert agricultural residues such as corn stover, distiller grains, wheat straw and sugar cane bagasse and energy crops such as switch grass into fermentable sugars which may be used to produce cellulosic ethanol.
Verenium (NASDAQ: VRNM), the new name of recently merged Diversa and Celunol Corporations, operates a pilot cellulosic ethanol plant in Jennings, Louisiana and is building a 1.4 million gallon per year demonstration plant on adjacent land to be completed by the end of 2007 and begin operation in early 2008. Vernium is the first publicly traded company with integrated, end-to-end capabilities to make cellulosic biofuels.
Traditionally, baker’s yeast (Saccharomyces cerevisiae), has long been used in brewery industry to produce ethanol from hexoses (6-carbon sugar). Due to the complex nature of the carbohydrates present in lignocellulosic biomass, a significant amount of xylose and arabinose (5-carbon sugars derived from the hemicellulose portion of the lignocellulose) is also present in the hydrolysate. For example, in the hydrolysate of corn stover, approximately 30% of the total fermentable sugars is xylose. As a result, the ability of the fermenting microorganisms to utilize the whole range of sugars available from the hydrolysate is vital to increase the economic competitiveness of cellulosic ethanol and potentially bio-based chemicals.
In recent years, metabolic engineering for microorganisms used in fuel ethanol production has shown significant progress . Besides Saccharomyces cerevisiae, microorganisms such as Zymomonas mobilis and Escherichia coli have been targeted through metabolic engineering for cellulosic ethanol production.
Recently, engineered yeasts have been described efficiently fermenting xylose  and arabinose , and even both together . Yeast cells are especially attractive for cellulosic ethanol processes as they have been used in biotechnology since hundred of years, as they are tolerant to high ethanol and inhibitor concentrations and as they can grow at low pH values which avoids bacterial contaminations.
Combined hydrolysis and fermentation
Some species of bacteria have been found capable of direct conversion of a cellulose substrate into ethanol. One example is Clostridium thermocellum, which utilizes a complex cellulosome to break down cellulose and synthesize ethanol. However, C. thermocellum also produces other products during cellulose metabolism, including acetate and lactate, in addition to ethanol, lowering the efficiency of the process. Some research efforts are directed to optimizing ethanol production by genetically engineering bacteria that focus on the ethanol-producing pathway.
Gasification process (Thermochemical approach)
The gasification process does not rely on chemical decomposition of the cellulose chain (cellulolysis). Instead of breaking the cellulose into sugar molecules, the carbon in the raw material is converted into synthesis gas, using what amounts to partial combustion. The carbon monoxide, carbon dioxide and hydrogen may then be fed into a special kind of fermenter. Instead of sugar fermentation with yeast, this process uses a microorganism named “Clostridium ljungdahlii” . This microorganism will ingest (eat) carbon monoxide, carbon dioxide and hydrogen and produce ethanol and water. The process can thus be broken into three steps:
- Gasification — Complex carbon based molecules are broken apart to access the carbon as carbon monoxide, carbon dioxide and hydrogen are produced
- Fermentation — Convert the carbon monoxide, carbon dioxide and hydrogen into ethanol using the Clostridium ljungdahlii organism
- Distillation — Ethanol is separated from water
A recent study has found another Clostridium bacterium that seems to be twice as efficient in making ethanol from carbon monoxide as the one mentioned above
Alternatively, the synthesis gas from gasification may be fed to a catalytic reactor where the synthesis gas is used to produce ethanol and other higher alcohols through a thermochemical process . This process can also generate other types of liquid fuels, an alternative concept under investigation by at least one biofuels company
Economic importance and viability
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Construction of pilot scale lignocellulosic ethanol plants requires considerable financial support through grants and subsidies. On 28th Feb 2007, the US Dept of Energy announced $385 million in grant funding to 6 cellulosic ethanol plants. This grant funding accounts for 40% of the investment costs. The remaining 60% comes from the promoters of those facilities. Hence, a total of $1000 million will be invested for approximately 140 million gallon capacity. This translates into $7/annual gallon in capital investment costs. This seems to be high because these are pilot plants; in the near future, we may expect the costs to be 2.5 - 4 times the capital costs of a corn ethanol plant. Corn to ethanol plants cost roughly $1 - $3/annual gallon capacity.
The quest for alternative sources of energy has provided many ways to produce electricity, such as wind farms, hydropower, or solar cells. However, about 40% of total energy consumption is dedicated to transportation (i.e., cars, planes, lorries/trucks, etc.) and currently requires energy-dense liquid fuels such as gasoline, diesel fuel, or kerosene. These fuels are all obtained by refining petroleum. This dependency on oil has two major drawbacks: burning fossil fuels such as oil contributes to global warming; and importing oil creates a dependency on oil producing countries.
Ethanol today is produced mostly from sugars or starches, obtained from fruits and grains. In contrast, cellulosic ethanol is obtained from cellulose, the main component of wood, straw and much of the plants. Since cellulose cannot be digested by humans, the production of cellulose does not compete with the production of food. The price per ton of the raw material is thus much cheaper than grains or fruits. Moreover, since cellulose is the main component of plants, the whole plant can be harvested. This results in much better yields per acre—up to 10 tons, instead of 4 or 5 tons for the best crops of grain.
The raw material is plentiful. Cellulose is present in every plant: straw, grass, wood. Most of these "bio-mass" products are currently discarded. Transforming them into ethanol using efficient and cost effective hemi(cellulase) enzymes or other processes might provide as much as 30% of the current fuel consumption in the US—and probably similar figures in other oil-importing regions like China or Europe. Moreover, even land marginal for agriculture could be planted with cellulose-producing crops like switchgrass, resulting in enough production to substitute for all the current oil imports.
In June 2006, a U.S. Senate hearing was told that the current cost of producing cellulosic ethanol is US $2.25 per US gallon (US $0.59/litre). This is primarily due to the current poor conversion efficiency. At that price it would cost about $120 to substitute a barrel of oil (42 gallons), taking into account the lower energy content of ethanol. However, the Department of Energy is optimistic and has requested a doubling of research funding. The same Senate hearing was told that the research target was to reduce the cost of production to US $1.07 per US gallon (US $0.28/litre) by 2012.
Prominent Cellulosic Ethanol Researchers
- Bruce Dale, Michigan State University
- 2007 USDA Sterling B. Hendricks memorial lecturer
- 1996 Charles D. Scott awardee 
- The Inventor of Ammonia Fiber Expansion (AFEX) pretreatment 
- Fourteen US international patent holder 
- Among the group of ten experts from industry, academia and government lab who was invited to brief President Bush on biofuels 
- Nancy Ho, Purdue University
- Mark Holtzapple, Texas A&M University
- Lonnie Ingram, University of Florida IFAS
- 2007 Charles D. Scott awardee
- Member of U.S. National Academy of Sciences (2001)
- Fellow of Society of Industrial Microbiology (2001)
- Twelve U.S. Patents holder including the landmark patent 5,000,000 on an important breakthrough on metabolic engineering of Escherichia coli to utilize virtually all sugars from lignocellulosic materials for ethanol production 
- Lee Lynd, Dartmouth College
- J.H. David Wu, University of Rochester
- Eckhard Boles, Goethe-University of Frankfurt
- Charles E. Wyman, University of California, Riverside
- Butanol fuel
- Cellulosic ethanol commercialization
- Non food crops
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- Klinke HB, Thomsen AB, Ahring BK (2004) Inhibition of ethanol-producing yeast and bacteria by degradation products produced during pre-treatment of biomass. Appl Microbiol Biotechnol 66:10-26
- Olsson L, Hahn-Hägerdal B (1996) Fermentation of lignocellulosic hydrolysates for ethanol fermentation. Enzyme Microb Technol 18:312-331
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- Wood Alcohol. Translation from E. Boullanger: Distillerie Agricole et Industrielle (Paris: Ballière, 1924).
- Lynd LR (1996) Overview and evaluation of fuel ethanol from cellulosic biomass: technology, economics, the environment, and policy. Annu Rev Energy Environ 21:403-465
- Iogen.ca process explanation. (2005)
- Sunopta Updates Current Cellulosic Ethanol Projects. Sunopta press release, 2007.
- Jeffries TW, Jin YS (2004) Metabolic engineering for improved fermentation of pentoses by yeasts. Appl Microbiol Biotechnol 63: 495-509
- Ohgren K, Bengtsson O, Gorwa-Grauslund MF, Galbe M, Hahn-Hagerdal B, Zacchi G (2006) Simultaneous saccharification and co-fermentation of glucose and xylose in steam-pretreated corn stover at high fiber content with Saccharomyces cerevisiae TMB3400. J Biotechnol. 126(4):488-98.
- Becker J, Boles E (2003) A modified Saccharomyces cerevisiae strain that consumes L-Arabinose and produces ethanol. Appl Environ Microbiol. 69(7):4144-50.
- Karhumaa K, Wiedemann B, Hahn-Hagerdal B, Boles E, Gorwa-Grauslund MF (2006) Co-utilization of L-arabinose and D-xylose by laboratory and industrial Saccharomyces cerevisiae strains. Microb Cell Fact. 10;5:18.
- University of Rochester Press Release: Genome Sequencing Reveals Key to Viable Ethanol Production
- Formation of Ethanol from Carbon Monoxide via New Microbial Catalyst,Biomass & Energy v. 23 (2002), p. 487-493.
- "DOE Selects Six Cellulosic Ethanol Plants for Up to $385 Million in Federal Funding". United States Department of Energy. 2007-02-28.
- "Feasibility Study for Co-Locating and Integrating Ethanol Production Plants from Corn Starch and Lignocellulosic Feedstocks" (PDF). United States Department of Energy. 2005-01. Check date values in:
- "Determining the Cost of Producing Ethanol from Corn Starch and Lignocellulosic Feedstocks" (PDF). U.S. Department of Agriculture and U.S. Department of Energy. 2000-10. Check date values in:
- "R-Squared Energy Blog". Retrieved November 19. Unknown parameter
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- Grain and cellulosic ethanol: History, economics, and energy policy, Biomass and Bioenergy, Barry D. Solomon, Justin R. Barnes and Kathleen E. Halvorsen (2007) In Press, Corrected Proof, Available online 13 March 2007.
- Ethanol From Cellulose: A General Review - P.C. Badger, 2002
- Cellulosic Ethanol Companies, News and Stock / Investor Information
- US DOE page on cellulosic ethanol production via enzymatic hydrolysis
- US DOE page on cellulosic ethanol production via synthesis gas fermentation
- Rocky Mountain Institute page on ethanol
- The "Wood-Ethanol Report" by Environment Canada, 1999, re-published by the Journey to Forever web site.
- US Senate committee hearing statement from Dr. Michael Pacheco, including current costs and expected costs of producing cellulosic ethanol.
- DOE Selects Six Cellulosic Ethanol Plants for Up to $385 Million in Federal Funding Feb 28, 2007
- The numbers behind ethanol, cellulosic ethanol, and biodiesel in the U.S. by Maywa Montenegro, Grist Magazine, 04 Dec 2006
- A list of cellulolytic bacteria